Genesis 1 as a whole describes the creation of the physical world, so it would be inconsistent to view the light as figurative rather than physical unless you read the whole chapter that way. On that basis, then, the light is physical, but what is its nature?
As summarized here, the talmud records a debate about the light in Chagigah 12a. One opinion is that the light of 1:3 is an intense light with special powers that God then restrained. The majority view, however, is that the light created in 1:3 is the same light that will ultimately be parcelled out to the sun, moon, and stars. Since the creation of these bodies on day 4 isn't accompanied by any additional creation of light, it is reasonable to assume that they used the pre-existing light.
Here is the passage from the g'mara on Chagigah 12a (Soncino translation and notes):
But was the light created on the first day? For, behold, it is written: And God set them in the firmament of the heaven,26 and it is [further] written: And there was evening and there was morning a fourth day27 — This is [to be explained] according to R. Eleazar. For R. Eleazar said: The light which the Holy One, blessed be He, created on the first day, one could see thereby from one end of the world to the other; but as soon as the Holy One, blessed be He, beheld the generation of the Flood and the generation of the Dispersion,28 and saw that their actions were corrupt, He arose and hid it from them, for it is said: But from the wicked their light is withholden.29 And for whom did he reserve it? For the righteous in the time to come,30 for it is said: And God saw the light, that it was good;31 and ‘good’ means only the righteous, for it is said: Say ye of the righteous that he is good.32 As soon as He saw the light that He had reserved for the righteous, He rejoiced, for it is said: He rejoiceth at the light of the righteous.33 Now Tannaim [differ on the point]: The light which the Holy One, blessed be He, created on the first day one could see and look thereby from one end of the world to the other; this is the view of R. Jacob. But the Sages say: It34 is identical with the luminaries;35 for they were created on the first day, but they were not hung up [in the firmament] till the fourth day.36
26) Gen. I, 17.
(27) Ibid., v. 19.
(28) I.e., the generation which built the Tower of Babel, and in consequence God confounded their language and scattered them over the earth. V. Gen. XI, 9.
(29) Job. XXXVIII, 15.
(30) I.e., the Messianic era; cf. Aboth II, 16.
(31) Gen. I, 4.
(32) Isa. III, 10. E.V. ‘that it shall be well with him.
(33) Prov. XIII, 9. E.V. ‘the light of the righteous rejoiceth.’
(34) I.e., the light created on the first day.
(35) V. Gen. I, 14f (E.V. ‘lights’).
(36) Cf. Gen. Rab. I, 14, and Rashi to Gen. I, 14.
Please note: This answer was written for a neutral, academic audience
and is not intended to be interpreted in the context of a religious
belief or doctrine.